1. Field of the Invention
This invention relates to a new process for preparing organic compounds containing the cyclopropane ring system, particularly substituted cyclopropanes having utility as pyrethroid insecticides or as intermediates in the preparation of pyrethroid insecticides, and to new compositions of matter useful in the practice of this process.
2. Description of the Prior Art
The class of pyrethroid insecticides includes both natural and synthetic members. The active natural products are extracted from the blossoms of pyrethrum flowers (Chrysanthemum cinerariaefolium) grown mainly in East Africa. The composition of the extracts has been elucidated by continuing the classical work of Staudinger [Helv. Chim. Acta, 7, 390 (1924)]. Harper [J. Chem. Soc., 892 (1946)], LaFarge et al. [J. Am. Chem. Soc., 69, 2932 (1947)], Godin et al. [J. Chem. Soc. (C), 3321 (1966)] as well as Crombie et al. [Chem. & Ind., 1109 (1954)] contributed to proving that the extracts comprise at least six closely related vinylcyclopropanecarboxylates: pyrethrin I, pyrethrin II, cinerin I, cinerin II, jasmolin I and jasmolin II. The most important natural pyrethroid is pyrethrin I. ##STR1## The structures of the other five components display variations in the portions of the molecule indicated by the arrows. In cinerin II and jasmolin II the dimethylvinyl group at the 2-position becomes (methyl) (carbomethoxy)vinyl; while in the cinerins the pentadienyl side chain in the alcohol moiety is 2-butenyl; in the jasmolins, 2-pentenyl.
In addition to optical isomerism, the pyrethroids display geometrical isomerism in that the hydrogen atoms at the 1 and 2 positions of the cyclopropane ring may be in either a cis or a trans relationship with respect to each other as illustrated in the drawing of the pyrethrin I molecule. The natural pyrethrin extracts comprise the trans forms and it is known that the trans isomers are more active. It is believed that there are two important centers in the pyrethroid structure which especially affect insecticidal activity, namely the substituted vinyl group in the acid moiety and the unsaturated side chain in the alcohol portion of the molecule. The vinyl group is believed to be the site for metabolic attack and detoxification by the insect; whereas the nature of the alcohol moiety is believed to influence the photooxidative stability [Elliott, Chem. & Ind., 978 (1974)].
With the discovery of the structure of the natural pyrethroids and extensions by Campbell et al. [J. Chem. Soc., 283 (1945)] of the work begun by Staudinger, it has been possible to produce synthetic pyrethroids.
Until recently, 1,1,1-trichloro-2,2-(bis-p-chlorophenyl)ethane (DDT) and 1,2,3,4,5,6-hexachlorocyclohexane (BHC) were widely used as insecticides. However, in view of the resistance of these materials to biodegradation and their persistence in the environment, new insecticides producing less environmental harm have been sought. Pyrethroids have long been of interest because they are active against a wide range of insect species, they display relatively low toxicity toward mammals, and they do not leave harmful residues. For example, pyrethrin I is more than 100 times as potent toward mustard beetles (Phaedon cochleariae) as DDT, but only 1/4-1/2 as toxic toward rats [Elliott et al., Chem. & Ind., 978 (1974); Nature, 244, 456 (1973); Chemical Week, Apr. 12, 1969, p. 57].
Although they possess a number of desirable characteristics, the natural pyrethroids undergo rapid biodegradation, they have poor photooxidative stability, their availability is uncertain, and it is costly to extract and process them. For a number of years efforts have been underway around the world to produce synthetic pyrethroid insecticides which would overcome these disadvantages. A notable recent development was the discovery of a dihalovinylcyclopropanecarboxylate (Structure II) having a toxicity toward insects more than 10,000 times greater than that of DDT, with an oral toxicity toward mammals similar to pyrethrin I [Elliott et al., Nature, 244, 456 (1973)]. Although Structure II, in which the alcohol moiety is 5-benzyl-3-furylmethyl, does not have exceptional photooxidative stability, Elliott et al. discovered that 3-phenoxybenzyl analogs (Structure III where X is halogen) were remarkably resistant to photooxidative degradation [Nature, 246, 169 (1973), Belgian Patents Nos. 800,006 and 818,811].
The objects of this application are to present processes for the synthesis of pyrethroids in which the cyclopropanecarboxylic acid moiety contains a dihalovinyl group in the 2 position and to describe novel compositions of matter useful in the practice of these processes. Accordingly, processes of this invention lead to esters of such acids which either are or may be converted readily into pyrethroid insecticides. The major advantage of this invention is to provide a convenient synthetic route to pyrethroids of the type represented by Structures II and III.
The early synthetic pyrethroids were compounds in which only the alcohol portion of the ester structure was varied. Synthetic pyrethroids representing this type of variation include allethrin and resmethrin which both contain the dimethylvinyl group of pyrethrin I, but in which the alcohol is allethrolone or 5-benzyl-3-furylmethyl alcohol respectively. Like the natural products, these pyrethroide degrade rapidly in air and light [Elliott et al., Nature, 246, 169 (1973)]. Processes for preparing such pyrethroids generally have begun with chrysanthemic acid, obtained either by the hydrolysis of natural pyrethroids or by the method of Staudinger [Helv. Chim. Acta, 7, 390 (1924)].
Only in recent years has the cyclopropanecarboxylic acid moiety, especially the vinyl group thereof, been modified synthetically. Prior to the present invention, the known methods for varying the nature of the substituents occupying the 2 position in the cyclopropane ring included the following:
(1) Chrysanthemic acid or a naturally occurring chrysanthemate may be subjected to ozonolysis to produce caronaldehyde [Farkas et al., Coll. Czech. Chem. Com., 24, 2230 (1959)]. The aldehyde may then be treated with a phosphonium or sulfonium ylide in the presence of a strong base, followed by hydrolysis [Crombie et al., J. Chem. Soc. (C), 1076 (1970); Brit. Patent No. 1,285,350]. Such a reaction sequence is shown below. ##STR2## The reaction may be utilized where X is an alkyl group and also where X is halogen [South African Patent No. 733,528; J. Am. Chem. Soc., 84, 854, 1312, 1745 (1962)]. The reaction has been employed to prepare ethyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropane-1-carboxylate, a precursor of Structures II and III. Whereas the ylide reaction proceeds in about 80% yield, the yield of aldehyde from the oxidation is typically only about 20%. The oxidative degradation originated as a tool for proof of structure and was never intended for large-scale preparative use. The oxidation alone requires many hours to complete because mild conditions must be used to minimize the possibility of a violent oxidation of the organic compound. An overall yield of 16% may not be unacceptable when the process is used in research, but it is much too low to be of practical commercial utility. In addition, the starting material is costly since, in essence, it is akin to the compound which is being prepared.
(2) The original Staudinger synthesis of chrysanthemic acid involved the reaction of ethyl diazoacetate with 2,5-dimethylhexa-2,4-diene followed by saponification of the ester [Helv. Chim. Acta, 7, 390 (1924)]. Carbene addition to an unsaturated carbon-carbon linkage has become a general reaction for the preparation of the cyclopropane ring system [Mills et al., J. Chem. Soc., 133 (1973), U.S. Pat. Nos. 2,727,900 and 3,808,260]. Such a reaction, illustrated below, has been employed in the preparation of pyrethroids and also ethyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropane-1-carboxylate, precursor of II and III [Farkas et al., Coll. Czech. Chem. Comm., 24, 2230 (1959)]. In preparing the latter, the starting material may be the mixture of pentenols obtained by the condensation of chloral with isobutylene. ##STR3## The conversion of the mixture of pentenols to 1,1-dichloro-4-methyl-1,3-pentadiene, short of the cyclopropanecarboxylate, is reportedly only about 50%. This, coupled with the fact that in the last step the production of the diazo ester and its handling are extremely dangerous on a large scale, seriously limits the utility of the process. Furthermore, it is estimated that, should a pyrethroid of Structure III become a major agricultural commodity, commercial production by this method of enough of the dihalovinylcyclopropanecarboxylate to satisfy the potential demand might exhaust the world supply of zinc.
(3) Julia has described a third general method capable of allowing the substituents in the 2 position of the cyclopropane ring to be varied [U.S. Pat. Nos. 3,077,496, 3,354,196 and 3,652,652; Bull. Soc. Chim. Fr., 1476, 1487 (1964)]. According to this method, illustrated below, an appropriately substituted lactone is first treated with a halogenating agent, opening the ring, followed by base-induced dehydrohalogenation, forming a cyclopropane. Even in the relatively uncomplicated case where the terminal substituents on the vinyl group are methyl and the product is ethyl chrysanthemate, the yield is only 40%. ##STR4##
Moreover, lactones of special interest, such as 3-(.beta.,.beta.-dichlorovinyl)-4-methyl-.gamma.-valerolactone are not readily available. Even 3-isobutenyl-4-methyl-.gamma.-valerolactone, from which ethyl chrysanthemate is made, requires a 3-step synthesis from 2-methylhex-2-en-5-one, including a Grignard reaction. Grignard reactions are difficult to carry out on a large scale and, in any case, could probably not be utilized without destroying a dihalovinyl group were it present.
Thus, the processes taught in the prior art for varying the nature of the substituents occupying the 2 position in the cyclopropane ring, particularly processes for introducing a 2-dihalovinyl group, suffer from a number of disadvantages, the most serious of which are:
(1) The yields of cyclopropanecarboxylates are too low for practical application in commerce;
(2) The starting materials are not readily available, requiring additional synthetic steps, adding to costs and increasing the price of the product beyond that which the market will bear;
(3) The processes all involve at least one reaction which is difficult and dangerous to carry out on a large scale, inviting the risk of fire or explosion.